fix: preserve receipt subscription ids

[Osraka]

Summary

• Add optional Receipt.subscription_id support for subscriptionId receipt metadata.
• Preserve subscriptionId when parsing receipts produced by another SDK and when formatting receipts back to Payment-Receipt.
• Add regression coverage for optional-field roundtrip and foreign receipt parsing.

This aligns pympp with the Rust SDK receipt behavior, where subscription receipts keep subscriptionId as a first-class optional field instead of silently dropping it.

Tests

• PYTHONPATH=/tmp/pympp-test-deps:src python3 -m pytest tests/test_parsing.py -q
• PYTHONPATH=/tmp/pympp-test-deps:src python3 -m ruff check src/mpp/__init__.py src/mpp/_parsing.py tests/test_parsing.py
• PYTHONPATH=/tmp/pympp-test-deps:src python3 -m py_compile src/mpp/__init__.py src/mpp/_parsing.py tests/test_parsing.py
• git diff --check